Conventionally, it has been known that the blood vessel is enlarged when carbon dioxide (CO2) is absorbed from skin of a living body. Carbon dioxide penetrates skin by simply contacting the skin, enlarges the blood vessel of the penetration site, improves the blood circulation, and therefore, exerts various physiological effects of reducing the blood pressure, easing loads on the heart, improving metabolism, promoting the elimination of pain substances and waste products, etc.
Therefore, carbonated spring bathing and the like have been used for treatments for high blood pressure, heart disease and arteriosclerosis or the like. Further, recently, as well as the medical purpose, carbon dioxide has received attention in terms of health enhancement and cosmetic promotion by the blood circulation promoting effect.
As an example of conventional techniques to cause carbon dioxide to be absorbed into the human body, used the most widely are bath agents that generate carbon dioxide by being immersed in hot water in a bath. When a bath agent is put into hot water, carbon dioxide trapped in the bath agent appears in hot water in bubble form, part of carbon dioxide is dissolved in the hot water and contacts skin of a bathing person, and it is thus intend to osmose carbon dioxide from the skin into the body so as to exert the physiological effects as described above.
Further, as the conventional technique for bringing a larger amount of carbon dioxide into contact with the human body, carbon oxide bathing apparatuses are known (for example, Patent Documents 1, 2 and 3).
This kind of apparatus is to dissolve carbon dioxide in water in high concentrations, and Patent Document 1 discloses a bath-agent carbon-dioxide dispersing apparatus comprised of a carbon-dioxide bath-agent dissolving portion 2 and a carbon dioxide dispersing apparatus 1 having micropores 4, where bubbles of carbon dioxide become minute by both of micropores 3 provided between the dissolving portion 2 and the dispersing apparatus 1 and the micropores 4 provided in the dispersing apparatus portion, the area such that carbon dioxide contacts water in the bath significantly increases while the contact time is increased, and the bath agent is further prevented from floating due to bubbles around the bath agent, and thus, is intended to continue dissipating carbon oxide inside water in the bath until the end.
Further, in Patent Documents 2 and 3 are described a bath microbubble generating apparatus and carbon dioxide combined shower bathing apparatus where a bath is connected to a carbon dioxide generating apparatus, and a large amount of carbon dioxide supplied from the carbon dioxide generating apparatus is jetted in fine-bubble form to be actively dissolved in bath water in the bath.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-20985    Patent Document 2: Japanese Laid-Open Patent Publication No. 2007-159902    Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-168859